DRAMs are nowadays present in most electronic devices. They usually form the main memory of microcontrollers or microprocessors. Generally speaking, DRAMs divide themselves into standalone DRAMs and embedded DRAMs (in the following, eDRAMs), depending on whether they are formed on an independent die, or on the same die of the microcontroller/microprocessor, respectively. The size and design constraints of eDRAMs are different from those of standalone DRAMs. In particular, the capacity of eDRAMs is usually smaller than the capacity of standalone DRAMs. For instance, while standalone DRAMs are nowadays in sizes of gigabytes and more, embedded DRAMs can be found in sizes ranging from a few hundred kilobytes upward.
Generally, the smallest basic block of each DRAM is provided with a storage element, which can take several forms but is mostly realized as a capacitor, and a transistor that allows or blocks access to the memory element. When the transistor allows access, the memory element can be written or read. When no access is granted, the DRAM is in retention mode. The transistor is usually named “select transistor.”
In standalone DRAM, the select transistor is generally rather long, so as to reduce leakage from the memory cell and reduce short channel effects. Nowadays, the select transistor is a 3D element, which is folded in a trench. So the footprint of a long channel is small. On the other side, in embedded DRAMs, such a 3D element is usually not provided. Thus, in eDRAMs, a select transistor must be used that provides a very low off current, thereby guaranteeing an appropriate retention time for the memory cell, while still having a short channel length to save area. In order to achieve such aim, the select transistor is usually structured so as to have a high threshold voltage. This can be obtained, for instance, via an appropriate doping profile. However, the high threshold voltage is a disadvantage when the transistor has to be made conductive, since the on current cannot achieve a level sufficient for fast operation. This is usually solved by using the select transistor in overdrive mode when the select transistor has to be closed, that is, has to be made conducting, in order to read/write the value stored within the storage element.
The use of an overdrive voltage is rather complex since it requires the availability on the circuit of the high voltage itself and the select transistor has to be made resistant to overdrive operation. In particular, since the transistor has to be used in overdrive mode, its dielectric gate has to be thicker than that of a “standard” transistor as used, for instance, in the rest of the circuit. This further requires additional manufacturing steps, which increase the cost of the embedded DRAM. Additionally, such a mode of operation slows down the opening and closing of the select transistor, which, in turn, slows down the operation of the embedded DRAM. Moreover, the increase of the driving voltage to an overdrive level substantially increases the power consumption of the embedded DRAM.